Method for making solid polymer electrolyte and uses thereof

ABSTRACT

A method for preparing a composite alkaline solid polymer electrolyte from polyvinyl alcohol (PVA) polymer, potassium hydroxide and water is disclosed. Said polymer electrolyte is reinforced with glass-fiber cloth to increase its mechanical strength, thermal stability and electrochemical stability. The glass fiber cloth matrix provides a stable interface between the cathode and the anode to reduce the short circuit problem when the battery discharges at high rate. The processes for polymer electrolyte are controlled by molecular weight of PVA polymer, the sequence of feeding in reactants, the weight proportions of reactants, the reaction time, the reaction temperature, and the drying conditions, i.e. under the specified conditions of relative humidity (RH), temperature and drying time. The resulting electrolyte exhibits ionic conductivity of 0.15 S/cm or better at room temperature and has high mechanical intensity and good electrochemical stability. This polymer electrolyte can replace the conventional PP non-woven fabric separator in primary and secondary alkaline batteries, metal-air battery, nickel-metal hydride battery, nickel-cadmium battery, nickel-zinc battery, fuel cells and capacitors. This composite PVA-GF polymer electrolyte can be used in thin-film alkaline battery system (thickness &lt;4 mm) to reduce the weight and thickness of battery, while significantly increasing the energy and power densities of the battery. It offers another opportunity for making thin and lightweight batteries to be used in future 3C electronic products.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a method for preparing compositealkaline solid polymer electrolyte from polyvinyl alcohol (PVA) polymer,potassium hydroxide and water, which is reinforced with glass-fibercloth (GF) to increase its mechanical strength, thermal stability andelectrochemical stability. This composite PVA-GF polymer film may beapplied in first and secondary thin-film alkaline batteries.

[0003] 2. Description of the Related Art

[0004] Prior literature indicates that polyvinyl alcohol (PVA) is apolymer linked by covalent bonds and hydrogen bonds. It is an amorphouspolymer material with low-crystallinity, rotational structure and goodflexibility that can block the conduction of electron. PVA ishydrophilic due to its hydroxide group, and has good compatibility withwater and potassium hydroxide that also have hydroxide group. Theinternal conduction of metal ion in PVA polymer is brought about by thestrong coupling interaction of metal ion and polymer backbone thatproduces temporary coordinate bonding, and subsequently the migration ofpolymer chain. PVA is a polymer material with diverse applications. Itis also low-priced and free of any environmental impact.

[0005] Fiber glass cloth (see FIG. 1) has similar compositions ofordinary glass. Both are inorganic oxide with silicon dioxide (SiO₂) asmain component. The glass material is hard and brittle. If it issubjected to high-temperature melting and drawn into glass yarns, itwill become flexible with tensile strength increasing by a dozen folds.When used for reinforcement, this material is usually in superfinefibrous state that offers strength and excellent thermal stability.Therefore regardless of the resulting product, there is no residualstress. The broad applications of glass fiber cloth are unparalleled byordinary glass.

[0006] Glass fiber as reinforcement material possess the followingproperties:

[0007] 1) High tensile strength which is twice that of steel wire havingthe same mass.

[0008] 2) Dimensional stability: Under maximum stress, its unitdimensions change by 3-4% only.

[0009] 3) High thermal resistance: It retains 50% of tensile strengthunder the temperature of 343° C.

[0010] 4) Superior corrosion resistance: It exhibits excellent corrosionresistance and brittleness property when in contact with the majority ofchemicals.

[0011] 5) Excellent fire proofing: It does not burn (generate heat), norsmolder (generate smoke).

[0012] PVA polymer electrolyte has extremely high ionic conductivityafter processing, but its mechanical strength is not as good as ordinaryseparators due to structural toughness. This inventor found in the studythat the addition of glass fiber cloth in the preparation of PVA polymerelectrolyte greatly improved its mechanical strength up to five timesthat of ordinary separators (see Table 1 and FIG. 3) without sacrificingits conductivity and with the activation energy for ion reaction greatlylowered (see Table 2). It also solved the contraction problem afterlong-term storage. Due to the high mechanical strength of glass fibercloth reinforced PVA polymer film, it is less prone to deformationduring processing, charging, discharging or packaging of battery. Underscan electron microscope, no pin hole was found on the surface of PVA-GFfilm. Thus when used in zinc-air fuel cell, it blocks the entry of zincion into the air in the cathode when the anode zinc discharges (seeTable 3), thereby preventing the occurrence of short circuit. Theinventor also found that the electrolyte dipped in PVA polymer was keptin gel state which helps address the leakage problem of battery broughtabout electrolyte seeping through separator. Moreover this polymerelectrolyte retains high conductivity and electrochemical stabilityunder high temperature. TABLE 1 Comparison of Physical Properties ofSeparators Property Draw Thickness Width Strength Stress Elongationspeed Type (mm) (mm) (kg) (kg/cm²) (%) Toughness (mm/min) PP/PE 0.17 101.0 57.3 25.6 25 200 separator PVA-GF 0.58 10 5.5 96.1 22.2 27.5 200polymer electrolyte PVA 0.48 10 0.4 8.1 457 182.8 200 polymerelectrolyte

[0013] TABLE 2 Comparison of Ionic Conductivity (σ) of PVA and PVA-GFFilm Electrolyte σ (S/cm) PVA-GF Electrolyte, PVA Electrolyte 40 μmthick Temp (° C.) (M.W: 70,000-80,000) (M.W.: 70,000-80,000) 30 0.15260.1588 40 0.1799 0.1599 50 0.1875 0.1615 60 0.1926 0.1683  70. 0.20610.1763 Activation energy (Ea) 4.020  2.219  (kJ/mole)

[0014] TABLE 3 Comparison of Discharge of Zinc-Air Fuel Cell withDifferent Separators Type Utilization (%) Composite PVA-GF PP/PE 0615Cellulose Discharge current polymer electrolyte separator separator 150mA (at C/10) 96.00 89.33 81.00 300 mA (at C/5) 90.16 77.50 82.66Theoretical capacitance 1500 1500 1500 (mAh)

SUMMARY OF THE INVENTION

[0015] The present invention provided a method for preparing compositealkaline solid polymer electrolyte from polyvinyl alcohol (PVA) polymer,potassium hydroxide and water, which is reinforced with glass-fibercloth (GF) to increase its mechanical strength, thermal stability andelectrochemical stability. This composite PVA-GF polymer film may beapplied in first and secondary thin-film alkaline batteries.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention can be more fully understood by referenceto the following detailed description and accompanying drawings, inwhich:

[0017]FIG. 1 is a microscopic drawing of glass fiber;

[0018]FIG. 2 is a structural diagram of alkaline polymer battery usingPVA-GF film of this invention as separator;

[0019]FIG. 3 is graph comparing the mechanical strength of differentpolymer films;

[0020]FIG. 4(a) is an AC resistance/impedance graph of PVA filmelectrolyte;

[0021]FIG. 4(b) is an AC resistance/impedance graph of PVA-GF filmelectrolyte;

[0022]FIG. 5 is the Arrhenius Plot of conductivity (σ) versustemperature (T) of PVA-GF polymer electrolyte of this invention;

[0023]FIG. 6(a) is the cyclic voltammetry of alkaline PVA and PVA-GFpolymer electrolyte of this invention at dipolar electrodes; itspotential scan rate was 1 mV/s, its scan range between −1.5-1.5V, andits working electrode 316 stainless steel;

[0024]FIG. 6(b) is the cyclic voltammetry of alkaline PVA-GF polymerelectrolyte of this invention under different temperature;

[0025]FIG. 7(a) is the voltage diagram of nickel-metal hydride secondarybattery during charge-discharge cycle using alkaline PVA-GF polymerelectrolyte of this invention as separator;

[0026]FIG. 7(b) is a diagram of single charge/discharge voltage ofnickel-metal hydride secondary battery using alkaline PVA-GF polymerelectrolyte of this invention as separator;

[0027]FIG. 8(a) is a voltage diagram of zinc-air battery discharging atthe rate of C/10 using alkaline PVA-GF polymer electrolyte of thisinvention as separator and comparison with the electrochemicalproperties of other commercially available separators;

[0028]FIG. 8(b) is a voltage diagram of zinc-air battery discharging atthe rate of C/5 using alkaline PVA-GF polymer electrolyte of thisinvention as separator and comparison with the electrochemicalproperties of other commercially available separators;

[0029]FIG. 8(c) is the discharge voltage diagram of zinc-air batteryunder different discharge currents with alkaline PVA-GF polymerelectrolyte of this invention as separator; and

[0030]FIG. 9 is a flow chart for the synthesis of PVA-GF solid polymerelectrolyte of this invention.

DETAILED DESCRIPTION OF THE INVENTION

[0031] Key technical points for the high-conductivity alkaline solidpolymer electrolyte of this invention are as follows:

[0032] 1) Have PVA and potassium hydroxide react with water separately;

[0033] 2) Add the potassium hydroxide solution to the PVA solutiondepending on the dissolution of PVA in water under controlledtemperature and time;

[0034] 3) Terminate the reaction depending on the reaction time and thedissolution of the mixture and then spread the polymer of differentquantity on carrier tray to obtain films of desired thickness;

[0035] 4) Control the film formation time, temperature and humidity tokeep proper water content in the polymer film; and

[0036] 5) Test the electrochemical properties of polymer film.

[0037] The preparation procedure and method are described in details asfollows:

[0038] 1) Selection and Pre-Treatment of Raw Materials

[0039] Use PVA of 80-99% purity with average molecular weight in therange of 2,000-120,000, and preferably between 2,000 and 5,000, ineither granule or powder form. Use potassium hydroxide of 85% puritywith molecular weight of 56 g/mole in either granule or powder form.

[0040] 2) Reaction Sequence

[0041] The ratio of reactants and reaction sequence will directly affectthe composition of polymer film and film formation. If the weightpercentage of PVA is too high, dissolution will become difficult andconductivity will drop; if the weight percentage of PVA is too low, filmformation might not occur. If the weight percentage of potassiumhydroxide is too high, the resulting poor structure will make filmformation difficult. If both of these materials are fed at the sametime, neither will dissolve. Thus the proportion and dissolutionsequence of the reactants are vital in the polymer film process. Thisinventor finds that mixing 10-20 wt % PVA with 50-60 wt % water underambient temperature and in a closed environment for approximately twohours will result in complete dissolution. At the same time, adding15-25 wt % potassium hydroxide to 10-20 wt % water under ambienttemperature and in a closed environment to undergo mixture anddissolution.

[0042] 3) Control of Polymerization Conditions

[0043] The temperature and time of polymerization reaction will affectthe water content of polymer film; the higher the water content, thehigher the conductivity. But polymerization will only occur underspecific temperature. Thus the control of polymerization time andreducing the loss of water are vital. This invention mixes thecompletely dissolved PVA solution and potassium hydroxide solution underambient temperature. At this time, white solid matter results. Mix itwith the solutions thoroughly and heat the solutions in closed containerunder 50-100° C. for about 30 minutes until that solid matter iscompletely dissolved. Cool the solution in atmosphere for about 10minutes. After the solution is cooled, spread the alkaline polymer fluidon the carrier to obtain film of desired thickness.

[0044] 4) Film Formation Conditions

[0045] Cut glass fiber cloth of proper size and lay it flat on thecarrier tray. Pour the viscous polymer solution into it and then put thecarrier tray into the temperature/humidity chamber under 30-80° C. and30-60 RH % (optimum conditions are 50-60° C. and 20-30 RH %) for about30-60 minutes until solid polymer film is formed. Then take out thecarrier tray and leave it in atmosphere for 30 minutes before removingthe film.

[0046] 5) Testing the Electrochemical Properties of Polymer Electrolyte

[0047] (1) Testing of Conductivity

[0048] Measure the resistance of solid polymer electrolyte with AutolabFRA AC impedance analyzer and dipolar stainless steel electrodes withfrequency scan between 1 MHz-0.1 Hz and amplitude of 10 mV. Also measurethe conductivity of the polymer electrolyte with Autolab FRA(σ=I/R_(b)×A). At the right side high-frequency area of the Nyquistplot, the impedance value where Z″ axis (capacitance) intersects with Z′axis at zero is the resistance (Z′=R_(b)) of polymer film (R_(b)).

[0049] (2) Testing of Electrochemical Stability

[0050] Use Autolab GPES to measure the cyclic voltammetry of polymerelectrolyte, other types of separators with PVA electrolyte. Thepotential range is −1.5-1.5V, the scan rate is 1 mV/s and stainlesssteel (SS-316) is used as working electrode.

[0051] (3) Testing of Electric Property of Battery

[0052] Assemble a zinc-air battery using the PVA-GF polymer electrolyteof this invention and a zinc electrode (−) and air electrode (+) (seeFIG. 2); the electrode area is about 6 cm² (2 cm×3 cm). Dischargecurrent is 50 mA, 100 mA and 200 mA respectively and compare theperformance of batteries with different separator. Also assemble asecondary nickel-metal hydride battery using PVA-GF polymer electrolyteof this invention with metal hydride (MH) and nickel hydroxide aselectrodes and carry out charge and discharge tests with 10 mA.

[0053] (4) Computation of Chemical Composition of PVA Film

[0054] Use weight difference method to compute the composition ratio ofPVA polymer electrolyte before and after reaction.

[0055] (5) Computation of Activation Energy

[0056] Graph log σ against 1/T in Arrhenius Plot to obtain gradient andcalculate activation energy.

σ=σ_(o) exp(−E _(a) /RT)   (1)

logσ=logσ_(o) −E _(o)/2.303R×1000×1/T   (2)

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT

[0057] The present invention is further depicted with the illustrationof embodiments.

Embodiment 1

[0058] Weigh accurately 8.0 g of polyvinyl alcohol (PVA) and 40 g ofwater and place them into reactor. Measure the weight of reactor withPVA, water and agitator in it and record it. Agitate for one hour underambient temperature until PVA is completely dissolved. Dissolve 12.5 gof potassium hydroxide (KOH) in 100 g of water and then pour it into thereactor. Raise the reactor temperature to 60-70° C. and control thepolymerization time to under 30 minutes. Measure the weight of reactorwith resulting polymer inside and record it, and spread viscous polymerof specific weight (about 5-10 g polymer solution) on glass fiber (GF)and place it in temperature/humidity chamber (control the humidity at30-40 RH % and temperature at 50-60° C.) for one hour. After that, takeit out and leave it in atmosphere for 30 minutes to one hour. Remove thepolymer film and weigh it to calculate its chemical composition afterdrying. Preserve the polymer film in zipper tape for electrochemicalanalysis. (Refer to FIG. 9).

Embodiment 2

[0059] Take the PVA-GF polymer film obtained in Embodiment 1. Measureits thickness with digital thickness gauge and its ionic conductivitywith Autolab FRA of Eco Chemie BV (dipolar stainless steel electrodes)(refer to FIGS. 4(a) and 4(b)). The composition of PVA:KOH:H₂O in thePVA-GF polymer film is found to be 30:30:40 wt %; molecular weight (MW)of PVA is 75,000-80,000; its thickness is 0.58 mm or 0.6 mm.

[0060] The cyclic voltammetry obtained with Autolab FRA (made inNetherlands) is as shown in FIGS. 6(a) and 6(b), from which it islearned that in comparison with PVA film under ambient temperature, thePVA-GF polymer electrolyte in this preferred embodiment did not undergoany oxidation and reduction reaction within working voltage stability of−1.4-1.4V, i.e. there was absence of Faradic current flow. PVA-GFelectrolyte exhibited better electrochemical stability than commerciallyavailable PP/PE separator (voltage stability of −1.0V-1.0V) andcellulose separator (voltage stability of −1.2-1.2V) with broader rangeof electrochemical voltage.

[0061] From FIG. 5, the conductivity of PVA polymer electrolyte underambient temperature of this preferred embodiment was 0.1526 S/cm, itsactivation energy for reaction was 2-4 kJ/mole, which is much lower thanthe activation energy of epoxy ethane polymer electrolyte (22-40kJ/mole). Table 2 displays the change of conductivity of PVA electrolyteand PVA-GF electrolyte under different temperature.

Embodiment 3

[0062] Take 2.5 g zinc gel consisting of 70 wt % zinc powder, PTFE andKOH as cathode and self-prepared air electrode as anode to assemblezinc-air batteries using PP/PE and cellulose as separator respectively.In addition, take the PVA-GF film electrolyte from Embodiment 1 hereinto replace the aforesaid PP/PE and cellulose separator in the assemblyof another zinc-air battery, and compare the discharge property ofdifferent batteries (see Table 4). Keep the theoretical capacitance ofthe batteries at 1,500 mAh and use discharge current of 50 mA, 100 mA,150 mA and 200 mA under ambient temperature. The results are as shown inFIGS. 8(a), (b) and (c). In FIG. 8(a) at the discharge rate of C/10, thedischarge time of zinc-air battery using PP/PE as separator was 8.9hours and its utilization rate was 89.33%; the discharge time ofzinc-air battery using cellulose as separator was 8.1 hours and itsutilization rate was 81%; and the discharge time of zinc-air batteryusing PVA-GF film cellulose of Embodiment 1 herein was 9.6 hours and itsutilization rate reached 96%.

[0063] The reason for the significant discrepancy in utilization ratewas that the PP/PE or cellulose used in commercially available alkalinebattery had pin holes in the size of 20-30 μm. When the batterydischarged, the zinc anode would expand after discharge and the zinc wasturned into zinc oxide (ZnO) of smaller density, which, due to expansionand squeeze of the electrode, would enter the other electrode along thepin hole and bring about short circuit. When the composite PVA-GF filmelectrolyte was used as separator, temporary coordination bond wasformed due to the dipole force generated between the polymer chain andions, and ions were conducted through the flexibility of chain. As aresult, the expansion of zinc electrode wouldn't lead to short circuitdue to the presence of PVA-GF. Thus PVA-GF has higher utilization ratethan conventional separators. TABLE 4 Testing of Zinc-Air BatteryAssembled with PVA-GF Electrolyte (Electrode area A = 6 cm²) Dischargecurrent Battery property 50 mA 100 mA 200 mA Discharge time (hr) 58.328.3 13.1 Actual capacitance (mAh) 2915 2830 2620 Theoreticalcapacitance 3000 3000 3000 (mAh) Utilization rate (%) 97.16 94.33 87.33

Embodiment 4

[0064] Take 0.3 g paste containing 70 wt % nickel hydroxide powder asanode, AB₅-type hydrogen storage alloy as cathode, and PVA-GF ofEmbodiment 1 herein as separator to assemble a nickel-metal hydridesecondary battery. Its theoretical capacitance was 50 mAh, and thebattery charged and discharged at 10 mA. The results are as shown inFIGS. 7(a) and 7(b). From Table 5, it is leaned that the battery kept80-90% efficiency after 10 cycles of charge/discharge and itsutilization rate was over 80%. In the analysis of electrical property,its charge cut-off voltage was 1.5V and its discharge cut-off voltagewas 0.9V. TABLE 5 Charge/Discharge Results of Zinc-Metal Hydride BatteryAssembled with PVA-GF Electrolyte (Electrode area A = 6 cm²) CycleCondition 1 2 3 4 5 6 7 8 9 10 Theoretical 50 50 50 50 50 50 50 50 50 50capacitance (mAh) Charge current 10 10 10 10 10 10 10 10 10 10 (mA)Discharge 10 10 10 10 10 10 10 10 10 10 current (mA) Charge time (hr)4.3 4.2 4.3 4.5 4.5 4.3 4.6 4.2 4.7 4.7 Discharge time 3.6 03.55 3.5 3.63.8 4.1 4.3 4.0 4.1 4.1 (hr) Discharge 36 35.5 35 36 38 41 43 40 41 41capacitance (mAh) Coulomb 83.7 84.5 81.4 80 84.4 95.3 93.4 95.2 87.287.2 efficiency (%) Utilization rate 72 71 70 72 76 82 86 80 82 82 (%)

[0065] While the invention has been described with reference to apreferred embodiment thereof, it is to be understood that modificationsor variations may be easily made without departing from the spirit ofthis invention, which is defined by the appended claims.

What is claimed is:
 1. A method for preparing solid polymer electrolytefrom polyvinyl alcohol, alkaline metal hydroxide and water, featuringthe steps of: (1) taking polyvinyl alcohol with molecular weight between2,000-120,000 that comprises 10-20% of the whole reactant by weight tomix with water 50-60% by weight under ambient temperature and in aclosed environment, and at the same time, taking alkaline metalhydroxide 15-25% by weight to mix with water 10-20% by weight underambient temperature and in a closed environment; (2) subsequently mixthe completely dissolved polyvinyl alcohol solution and alkaline metalhydroxide solution together under ambient temperature and heat themixture in a closed container up to 50-100° C. to let it undergocopolymerization, and then leave it in atmosphere to cool; and (3)spread the cooled polymer on the carrier tray and then place the tray intemperature/humidity chamber for 30-60 minutes under the temperature of40-80° C. and humidity of 20-50 RH % to turn it into solid state polymerfilm.
 2. A method for preparing solid polymer electrolyte from polyvinylalcohol, alkaline metal hydroxide and water according to claim 1,wherein the reaction time in step (2) is 20-30 minutes.
 3. A method forpreparing solid polymer electrolyte from polyvinyl alcohol, alkalinemetal hydroxide and water according to claim 1, wherein the preferredconditions for the temperature/humidity chamber in step (3) are 50° C.in temperature and 20-30 RH % in humidity.
 4. A method for preparingsolid polymer electrolyte from polyvinyl alcohol, alkaline metalhydroxide and water according to claim 1, wherein said polyvinyl alcoholpreferably has average molecular weight of 2,000-50,000.
 5. A method forpreparing solid polymer electrolyte from polyvinyl alcohol, alkalinemetal hydroxide and water according to claim 1, wherein said alkalinemetal hydroxide may be KOH, NaOH, or LiOH or its mixture.
 6. A methodfor preparing solid polymer electrolyte from polyvinyl alcohol, alkalinemetal hydroxide and water according to claim 1, wherein said glass fibercloth has a thickness of 10-600 μm.
 7. A method for preparing solidpolymer electrolyte from polyvinyl alcohol, alkaline metal hydroxide andwater according to claim 1, wherein the water content of said solidpolymer electrolyte is between 20-50 wt %, and its optimum chemicalcomposition is between 30-40%; wherein such composition gives theresulting polymer electrolyte higher conductivity with the formation offree standing film.
 8. A method for preparing solid polymer electrolytefrom polyvinyl alcohol, alkaline metal hydroxide and water according toclaims 1-7, wherein the resulting solid polymer electrolyte may be usedin nickel-metal hydride battery, nickel-cadmium battery, nickel-zincbattery, fuel cells, metal-air battery, primary and secondary alkalinebatteries or alkaline capacitors.